The present invention relates generally to semiconductor devices, and, more specifically, to a method of wire bonding in semiconductor devices.
Wire bonding is the most commonly used semiconductor device electrical interconnect technology. Wire bonding is a solid phase welding technique that uses a combination of heat, pressure, and ultrasonic energy to form a bond between an interconnect wire and a bond pad surface through atomic diffusion and electron sharing.
Earlier wire bonding technology used gold wires for forming electrical connection. In recent years, copper has emerged as viable substitute for gold wires. Copper wire bonding not only provides significant cost benefits but also offers several mechanical and electrical advantages. However, use of copper in wire bonding also poses several challenges. Copper is harder than gold, and thus, a copper wire needs greater mechanical force and ultrasonic power to form a bond with the die bond pads.
It is well known that the copper wire bonding process subjects the semiconductor device to significant thermal and mechanical stress, which can damage the layers of metal and dielectric underlying the die bond pad, also known as silicon cratering. Cratering adversely affects the quality and reliability of a wire bond. The major factors responsible for the damage include the mechanical impact of the capillary during the ball bonding process; ultrasonic vibration frequency and energy; and process temperature.
One of the emerging trends in semiconductor fabrication is the use of dielectric materials with low dielectric constants. Examples of such materials include silicon-containing hydrogen compounds, aero-gels, and organic compounds. Such materials are preferable as they help to reduce the effective capacitances, and thus, facilitate achieving increased speed of processing in semiconductor devices. However, such materials are mechanically weaker than the conventional dielectric materials (deposited using chemical vapor deposition techniques). Accordingly, use of such materials in semiconductor devices greatly aggravates the cratering problem discussed above.
Another important factor leading to failure of wire bonds is excessive bond pad probing. The semiconductor chip is subjected to various electrical tests using probes. Probe testing leaves probe marks or scratches on the bond pads that deform the surface topography of the bond pad, making it uneven and rough. The probe tip can even cause the bond pad under-layers to be exposed. Thus, probe marks hinder effective inter-metallization between the wire and the bond pad and accordingly, adversely affect the integrity of wire bonds.
Copper also is used to form the chip bond pads. However, copper has a tendency to oxidize, forming copper oxide compounds, which hinder inter-metallization during wire bonding. To overcome this problem, the industry practice is to cap copper bond pads with aluminum. However, as discussed above, probe tips can gouge the surface of the bond pad such that the aluminum upper layer is scratched and the underlying copper layer is exposed, which leads to the formation of copper oxides.
It also is desirable to reduce the thickness of the chip bond pads. However, bond pads with reduced thickness cannot bear the mechanical impact generated during the wire bonding process and are sometimes damaged. Therefore, another important challenge is to be able to reduce the bond pad thickness and yet obtain a reliable wire bond with sufficient mechanical strength.
Further, the number of bond pads is steadily increasing, the bond pad pitch is being reduced, and the individual bond pads are becoming much smaller and consequently, the area of contact available for bonding is shrinking. With the shrinking sizes of the bond pads, the area of the bond pad that is damaged by the probe tip is increasing, causing greater and greater hindrance to crimping and alloy formation during bonding process. Thus, the pad damage due to the probe marks leads to wire bond failure conditions, such as non-stick on pad and lifted bonds.
It would be advantageous to be able to reliably bond copper wire to small, thin bond pads.